The present invention relates to a nuclear fuel assembly with a substantially square cross section for a light water reactor comprising a plurality of fuel rods extending between a top tie plate and a bottom tie plate.
In a nuclear reactor, moderated by means of light water, the fuel exists in the form of fuel rods, each of which contains a stack of pellets of a nuclear fuel arranged in a cladding tube, a column of extruded fuel cylinders or an uninterrupted column of vibration-compacted powdered fuel. The cladding tube is normally made of a zirconium-base alloy. A fuel bundle comprises a plurality of fuel rods arranged in parallel with each other in a certain definite, normally symmetrical pattern, a so-called lattice. The fuel rods are retained at the top by a top tie plate and at the bottom by a bottom tie plate. To keep the fuel rods at a distance from each other and prevent them from bending or vibrating when the reactor is in operation, a plurality of spacers are distributed along the fuel bundle in the longitudinal direction. A fuel assembly comprises one or more fuel bundles, each one extending along the main part of the length of the fuel assembly. Further, the fuel assembly comprises one or more centrally arranged channels which conduct non-boiling water through the fuel assembly to improve the moderation in the central parts of the fuel assembly.
The core is immersed in water which serves both as coolant and as neutron moderator. During operation, the water flows from below and upwards through the fuel assembly, whereby, in a boiling water light-water reactor, part of the water is transformed into steam. The percentage of steam increases towards the top of the fuel assembly. Consequently, the coolant in the lower part of the fuel assembly consists of water whereas the coolant in the upper part of the fuel assembly consists both of steam and of water. This difference between the upper and lower parts gives rise to special problems which must be taken into consideration when designing the fuel assembly.
It is therefore desirable to achieve a flexible fuel assembly for a boiling water reactor which, in a simple manner, may be given a shape in which the upper part of the fuel assembly differs from the lower part thereof. A fuel assembly for a boiling water reactor with these properties is shown in PCT/SE95/01478 (Int. Publ. No. WO 96/20483). This fuel assembly comprises a plurality of fuel units stacked on top of each other, each comprising a plurality of fuel rods extending between a top tie plate and a bottom tie plate. The fuel units are surrounded by a common fuel channel with a substantially square cross section. A fuel assembly of this type may in a simple manner be given different designs in its upper and lower parts.
Also in a light-water reactor of pressurized-water type, it may be desirable to design the fuel assemblies such that each fuel assembly comprises a plurality of fuel units stacked on top of each other. As described above, each one of the fuel units then comprises a plurality of fuel rods extending between a top nozzle and a bottom nozzle. A fuel assembly for a pressurized-water reactor, however, comprises no fuel channel.
One factor which must be taken into consideration when designing such fuel units for boiling water reactors is that the fuel rods must be kept at a distance from the fuel channel which surrounds them. In the same way, the fuel rods in a fuel assembly for a pressurized-water reactor must be kept at a distance from an adjacently located fuel assembly. Normally, these distances are achieved by designing spacers for retaining and positioning the fuel rods in relation to each other with members therefor. In a fuel assembly for a boiling water reactor, these members make contact with the fuel channel and in a fuel assembly for a pressurized-water reactor, similar members make contact with a spacer provided in an adjacently located fuel assembly. In those cases where shorter fuel units of the above-mentioned type are used, spacers are not necessary, so the spaced relationship must be achieved in some other way.
It is also common to arrange some form of mixing vanes for the spacers. Since the need of spacers is eliminated, these must be achieved in some other way.
In CA 1 250 966, a fuel unit is shown in Which the need of spacers is eliminated. The fuel unit is intended to be used in a reactor moderated with heavy water with pressure tubes. The fuel units and the pressure tubes have a circular cross section. The fuel rods are arranged between a pair of end plates. To ensure that the fuel rods are kept at a distance from the pressure tubes, the end plates are provided with beams extending radially out from the respective end plate and in a direction parallel to the direction of flow and between the fuel rods. The beams make contact with the inner surface of the pressure tube and thus keep the fuel rods at a distance therefrom. One disadvantage of this embodiment is that it is complicated to manufacture since the beams must be joined to the end plates in some way, for example by welding or soldering. Further, the end plates are designed to make tight contact with each other or be welded together to minimize the pressure drop of the coolant across the end plates. One disadvantage of this method is that the fuel rods in this region are not given a possibility of growing differentially because of thermal expansion during operation of the reactor. The problem with differential growth, however, is not so great in this type of reactors since the burnup of the fuel assemblies is limited.
An object of the present invention is to provide a fuel assembly with a plurality of short fuel units comprising top and bottom tie plates, respectively, provided with members which are adapted to keep the fuel rods at a distance from a surrounding, substantially rectangular, fuel channel or an adjacently arranged fuel assembly which is simple to manufacture.
It is another object of the invention to provide a fuel assembly with mixing vanes which are simple to achieve in the- fuel assembly.
A further object of the invention is to provide a fuel assembly with members which permit differential growth of the fuel rods.
The present invention relates to a fuel assembly with a substantially square cross section for a light-water reactor comprising a plurality of fuel rods arranged between a top tie plate and a bottom tie plate.
The fuel assembly comprises a plurality of fuel units, each comprising a plurality of fuel rods extending between a bottom tie plate and a top tie plate. The bottom tie plate and the top tie plate may be identically designed. According to one aspect of the invention, the bottom tie plate and the top tie plate are manufactured by punching in a sheet. The punched piece is provided with side supports which, prior to mounting to the fuel rods, are folded approximately 90xc2x0 around an axis parallel to the plane of the top tie plate and the bottom tie plate, respectively. The task of the side supports, in a fuel assembly for a boiling water reactor, is to make contact with the fuel channel which surrounds the fuel rods to keep the fuel rods at a distance from the fuel channel. In a fuel assembly for a pressurized-water reactor, the task of the side pieces is to make contact with adjacent bottom and top tie plates, respectively, in order thus to keep adjacently located fuel assemblies at a distance from each other.
The top tie plates and the bottom tie plates, respectively, are made of sheet metal which is so thin that it is allowed to deform because of the differential growth of the fuel rods. It is suitable to choose a thickness of the sheet of the order of magnitude of 0.7 millimetres.
In one embodiment of the invention, the bottom tie plates are preferably provided with tabs during the punching, which tabs, prior to mounting, may be folded into the desired angle and shape for achieving mixing vanes. The task of the mixing vanes is to mix the coolant flowing upwards through the fuel assembly to increase the cooling capacity of the coolant. Increased cooling capacity is desirable particularly in the upper part of the fuel assembly, where the coolant has a higher temperature.
In a further embodiment of the invention, in connection with or after the punching, the bottom and top tie plates, respectively, are provided with a folded cross section. The folded cross section results in two adjoining top and bottom tie plates making contact with each-other in portions formed between the fuel rods and in the creation of a space between the top and bottom tie plates in those portions where the fuel rods are arranged. This space allows differential growth of the fuel rods because of thermal expansion by the top and bottom tie plates, respectively, being locally deformed into the space. In this way, the risk of fuel rods bending because of differential rod growth, hence preventing the full effect of the coolant, is reduced.
An advantage of the fuel assembly according to the invention is that it comprises top and bottom tie plates with side supports which are simple to manufacture from a plane or profiled sheet. The top and bottom tie plates may also be provided in a simple manner with mixing vanes and the desired cross section in one and the same work operation.
By forming the top and bottom tie plates, respectively, in thin sheet metal, only a small space is obtained between two fuel units stacked on top of each other. This is an advantage in that power peaks, which otherwise may arise in axial gaps without fissionable material, are reduced.